The Nature of Minerals

Minerals are natural inorganic products that possess a characteristics
chemical composition and usually a definite crystal structure. All matters
are composed of one chemical element or a combination of elements; for
example, diamond is composed of carbon, a single element, whereas all
other gemstones are composed of combinations of elements. Metals are
usually single elements, but alloys used in jewelry are intimate mixtures
of metallic elements. The various mineral species are classified according
to their chemical composition and the arrangement of the atoms that
comprise them. In general, if a mineral formed under reasonably favorable
conditions the atoms of the elements that make up the mineral will have
arranged themselves in an orderly fashion, resulting in a definite and
characteristic internal structure. This definite structure is known
as a CRYSTALLINE STRUCTURE or a CRYSTAL STRUCTURE. If, on the other
hand, the substance is formed rapidly, or for some other reason in a
Manner that did not permit the atoms to arrange themselves in an orderly
fashion, the material is called AMORPHOUS (pronounced ah-MOR-fuss),
meaning without form. Amber, jet, glass and opal are amorphous. The
difference in internal arrangement of atoms in any single plane in crystalline
and amorphous materials might be likened to the difference between a
battalion of soldiers at attention and a crowd of people standing in
a field to watch a spectacle.

The study of crystal structure was once of interest only to mineralogists.
However, its paramount importance in revealing the nature of solid substances
has since become evident to scientists in almost every field. The first
use of X-ray diffraction by Von Laue early in the 20th century, and
the subsequent research into the nature of crystals, awakened scientists
to the fact that, in such research lay the answers to many of their
questions. In fact, studies that began on crystals led directly to the
study of the atoms themselves, which has opened many new fields, including,
of course, atomic energy.

The reason that we, concerned as we are with the practical uses of
a knowledge of gemstones, bother with a brief survey of the nature of
crystals will soon become clear. It has been found that a direct relationship
exists between the structure and the important properties that lend
beauty and durability to a gemstone. In addition, these properties are
important in cutting and vital to the identification of gem materials.
Since the crystal shapes in which gemstones often, occur determine the
style in which they must be cut to yield maximum weight and beauty,
the cutter must know crystals to estimate how much weight he can obtain
from a parcel of rough stones to bid on it intelligently. He must also
know enough about the properties of the stone to be able to cut it to
maximum advantage in terms of the ultimate beauty the rough can yield.
To make such estimates he must have a thorough knowledge not only of
the crystals but of crystal structures of gemstones. A study of the
various colored stones shows that the rough crystal shape and vital
differences of color with direction require that certain gemstones such
as ruby, emerald and alexandrite be cut in certain specific relationships
to the rough crystal directions. Failure to do so means a great loss
in beauty and value. Unfortunately, many of the stones seen in the trade
have been cut incorrectly with a resultant loss in per carat value.
The competent gemologist is often able, to correct such an error and
enhance the stones value by having it re-cut properly.